1. Field of the Invention
The present invention relates to a vertical die casting machine, i.e., a so called "Vertical Squeeze Casting machine", in which machine a molten metal or melt in a casting sleeve is injected by actuating an injection plunger into a cavity defined by a mold arrangement of a vertically clamping type including lower stationary and upper movable molds through a runner hole formed in the stationary mold, and the melt is poured into the casting sleeve while the sleeve is spaced apart from the stationary mold.
2. Description of the Related Art
Machines such as the above are disclosed in U.S. Pat. No. 4,088,178 and U.S. Pat. No. 4,287,935, having two of the three inventors named in the present invention. Such known machines of the vertically clamping type have the following inherent problems:
The casting or injection sleeve for use in the vertical die casting machine has a hollow head portion defining, with the head of the plunger therein, a melt space where the melt is received. The sleeve head portion has the same diameter over the length thereof and is not constricted at the free end thereof, which end abuts against the stationary mold from below. A melt passage from the surface of the melt received in the sleeve to the cavity is formed to include an enlarged runner hole. The injection of the melt for producing a cast product in the cavity is carried out in two stage operations. At the initial injection stage, the plunger is forced to move at a relatively high speed so that the melt is filled in the cavity. As the final injection stage, the plunger is actuated to move upward at a relatively low speed with an additional short stroke to have the melt filled in the cavity subjected to an increased pressure by the plunger. When the melt is injected into the cavity through the melt passage and then cooled, the solidified melt or cold melt forms, in an integral body, a cast product in the cavity, a melt part, i.e., "runner", in the runner hole, and the remaining melt part, i.e., "bisket", in the upper end portion of the sleeve. The enlarged runner hole is required to allow the bisket to pass through the runner hole so that the entire solidified melt is removed from the stationary mold when the upper movable mold is moved upward and separated from the lower stationary mold. In this regard, the diameter of the runner hole must be not less than that of the sleeve head.
One of the problems resides in that such enlarged runner hole occupies a substantial area of the lower surface of the cavity on the side of the lower stationary mold, since the runner hole opens to the lower surface. This occupation restricts a degree of freedom in designing contoured decorations to be formed at the lower surface of a cast product integrated with the melt runner.
The other problem resides in that such an enlarged runner hole causes the quality of a cast product to be reduced, for the following reason. After the hot melt is poured in the melt space in the sleeve, the received melt is partially cooled at the circumferential inner surface of the sleeve and the surface of the plunger tip to form a semi-solidified part of the melt along the circumferential surface before the injection, i.e., a "shell" of a cylindrical vessel shape. When the melt is injected into the cavity, the shell is broken into pieces and a substantial amount of the shell is forced to enter the cavity, accompanying the remaining hot melt part.
In order to avoid this intrusion of the shell, it has been attempted to provide the stationary mold with a detachable metal net covering the entire cross sectional area of the runner hole therein, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 55-42116. Alternatively, in another prior art, the upper end portion of the injection sleeve is provided with a ring detachably mounted therein, as shown in Japanese Examined Patent Publication (KOKOKU) No 56-5621. However, it is noted that these prior art solutions cause difficulties in manual handling of the net or the ring at each injection cycle. Further it is recognized that such means are likely to cause the production of a cast product to require an increased amount of the melt to be wasted as a non-product material.